Many devices, such as turreted artillery, aircraft landing gear, various kinds of reciprocating machinery, vehicle shock absorbers and struts, seismic event attenuation devices, etc., undergo or isolate severe impulse loading, that is high loading over very short durations. Proper handling of these loading conditions typically is essential to the survival, if not the proper functioning of the device. For example, the accuracy of stabilized turreted, rapid-fire gun systems is limited by the structural flexibility of the gun barrel and the gun mounting structure. To improve the accuracy of sustained rounds, high frequency recoil forces that excite the structural dynamics of the turret must be dissipated. Although artillery applications are referred to prominently herein, the principles and embodiments of the invention described below apply to any application with respect to which severe impulse loading is of concern.
Referring to FIG. 1, some high-caliber, rapid-fire guns G employ damping systems D to damp recoil forces transmitted to the gun mounting structure, or fork F, along a direction T that is generally aligned with gun trajectory. Typically, damping systems D rely on passive dampers.
As shown in FIG. 2, a passive damper 10 typically includes a cylinder 15, having a chamber that contains a working fluid. A piston 25 has a head 30, received in chamber 20, and a piston rod 35 extending from head 30 and through an aperture 40 in cylinder 15. The head 30 is moveable within the cylinder between ends 31 and 32, and typically has apertures or valves (not shown) that pass working fluid as head 30 moves against the working fluid. Alternatively, head 30 and chamber 20 may define a narrow passage (not shown) through which the working fluid passes.
Cylinder 15 defines a first eye 45, or other mounting convention, for installation to fork F. Piston rod 35 terminates in a second eye 50, or other mounting convention, for installation to gun G. A first spring retainer 55, connected to cylinder 15, and a second spring retainer 60, connected to piston rod 35, retain a recoil spring (not shown in FIG. 2, but see recoil spring 165 in FIG. 6) that biases piston 30 relative to cylinder 15 into a battery position.
When gun G discharges, gun G recoils with a force that urges piston 30 and cylinder 15 to translate relatively, against a restoring force of the recoil spring 62 and the viscous force of the working fluid against which piston 30 works. As piston 30 works against the working fluid, the working fluid becomes heated in an amount corresponding to the work. Thus, the energy associated with a recoil force is converted into or dissipated in the form of heat.
Energy dissipation directly corresponds to the viscosity of the working fluid. Viscosity is a measure of the resistence of fluid to angular deformation. That is, as viscosity or fluid resistence increases, the amount of work which a piston must undertake to move relative to the associated cylinder increases. Increasing the work that the piston exerts against the fluid increases the heat content or temperature of the fluid. The amount of heat generated and dispersed by the working fluid directly corresponds to the amount of recoil energy dissipated. In other words, increasing the viscosity of the working fluid which, during recoil, causes the piston to generate more heat in the working fluid, results in dissipating more energy of the recoil.
If the amount of energy a damper dissipates is too little, gun G recoils against forks F with an impact that can distort the forks F, adversely effecting gun accuracy, and can damage the forks F, associated electronics and other non-isolated physical structures. Large loads not damped, but transferred to, for example, the frame of a helicopter or other mobile gun transport, also will adversely impact transport handling properties or render the transport unstable or uncontrollable. If the amount of energy dissipated is too much, the gun recoil may be insufficient to compress the recoil spring, which in turn may prevent the gun from returning to the battery position. If gun G does not return to the battery position, gun G may not be able to expel spent cartridges, receive a new round or may experience other failures. Accordingly, energy dissipation must be carefully managed or predicted so that gun G is more accurate and does not prematurely breakdown due to inadequate recoil energy dissipation, or fail due to overly aggressive energy dissipation.
Passive dampers can not adequately damp guns because the amount of energy which passive dampers dissipate generally remains constant, whereas the recoil energy varies. A typical passive damper employs a working fluid that has a generally fixed or predictable viscosity. Fixed viscosity results in generally constant energy dissipation. Accordingly, a working fluid selected for a passive damper may be appropriate for damping a minimum anticipated recoil energy. In order to ensure that a recoil spring returns a gun to battery position. The amount of damping provided in such arrangements generally falls well short of most recoils realized. Consequently, less than an optimal amount of recoil energy is dissipated by the fluid. On the other hand, the amount of recoil energy realized varies according to factors such as round temperature, age, production facility, etc. Consequently, guns and gun mounts experience higher recoil forces than necessary, which introduces structural instabilities that adversely impacts accuracy. Guns and gun mounts also wear much faster than if equipped with more effective damping.
Although not in the context of artillery, dampers exist that provide for varying damping. Some variable dampers include actuated valves for controlling, thereby impacting effective damping, of the damper. However, these dampers rely on moving components to adjust damping, which is cumbersome and not readily adaptable to rapid extreme impulse loads.
Other variable dampers eliminate the mechanical viscosity control components by utilizing active working fluids having viscous properties that change under the influence of electric or magnetic fields. Active fluids, such as Magnetorheological (MR) and Electrorheological (ER) fluids, have the unique ability to change properties when electric or magnetic fields are applied thereacross, respectively. This change mainly is manifested as a substantial increase in the dynamic yield stress, or apparent viscosity, of the fluid.
MR fluids are preferred because of their superior performance. For example, as compared to ER fluids, MR fluids possess an order of magnitude higher yield stress and a much wider operating temperature range. Specifically, the COTS MR fluid, VersaFlo(trademark) by the Lord Corporation, is far less sensitive to contaminants than ER fluids and can be operated in a temperature range from xe2x88x9240 to 150 degrees Celsius. A key advantage of MR fluids is that they require activation voltages of less than 100 volts, an order of magnitude less than ER fluids. This low-voltage operation capability is particularly attractive where heavy power amplifiers cannot be accommodated. In summary, the advantages of MR fluids derive from their ability to provide robust, rapid response interfaces between electronics controls and mechanical systems in real time.
MR devices, such as rotary brakes and linear displacement dampers have been commercialized. However, while the overall use of MR fluid in these devices has increased, both in terms of effectiveness and creativity, the analytical modeling and systematic design aspects have lagged. To a large extent, this can be attributed to the complex phenomenological behavior of these fluids.
MR fluids exhibit nonlinear effects due to applied field, applied load, strain amplitude, and frequency of excitation in dynamic displacement conditions. FIG. 3A is a schematic drawing of the COTS Lord Rheonetics(trademark) damper, white FIG. 3B shows representative test data obtained from this device. The plots show the force vs. piston displacement and force vs. velocity behavior of typical MR damper designs as a function of applied field. The total energy dissipated by the damper is represented by the area within the hysteresis cycles on the force vs. displacement plot in FIG. 4. As greater excitation voltages are applied, more energy is dissipated by the MR damper. This hysteretic response, in addition to the variable damper yield force, as shown in the force vs. velocity plots in FIG. 5, may be exploited in a full-scale flow mode damper for large, rapid fire guns to dissipate energy and to damp the dynamic response of the gun system.
Like most MR and ER dampers available, the COTS Lord Rheonetics(trademark) damper provides constant field excitation, for constant damping control, rather than variable, rapidly controllable, adaptive excitation field control for optimal damping. Consequently, COTS Lord Rheonetics(trademark) dampers, although tunable to trace any of the hysteresis curves, when employed in a device, can only trace one of the hysteresis curves due to a constant applied field.
In the development of the analysis of the recoil adapters, some consideration must be made as to the complexity of the underlying fluid mechanics analysis. The magnetorheological (MR) fluids to be used in the adapters are composed of a suspension of micron sized iron particles in a carrier fluid, typically silicone oil. In the following discussion, it must be realized that the physics of the flow through an MR damper are straightforward: high shear rate Poisieulle flow through an annular valve. The annular valve can be simplified to a rectangular valve using a small ratio assumption, that is, the ratio of the gap to the radius of the annular valve is small or
xe2x80x83d/r less than  less than 1xe2x80x83xe2x80x83(1)
Thus, three options exist for developing an analysis of the flow through the annular valve: (1) particle interaction models, (2) continuum models, and (3) rheological models. The particle interaction models have a high computation load, thus are not helpful in modeling this complex. The continuum models only pertain to pre-yield behavior, thus are not particularly helpful in controlling a system that yields. However, the rheological models seem to be most useful for this application because such treat the fluid in bulk, rather that as individual particles; and relate the shear stress to the shear rate.
The three most useful rheological models are: (1) Bingham-plastic, (2) Herschel-Bulkley, and (3) an Eyring-Prandtl-Re constitutive model. The first two models produce relationships between damper velocity and damper force. These models are limited to quasi-steady conditions. However, further research will lead to extending these models to a broader range of conditions. The Eyring model only allows for velocity to be expressed in terms of the force, and thus is not as useful a tool for as the other mentioned models. It is useful to summarize these models and to describe their deficiencies.
The Bingham-plastic constitutive model can be expressed as:
xcfx84=xcfx84ysgn({dot over (xcex3)})+xcexc{dot over (xcex3)}xe2x80x83xe2x80x83(2)
A key point is that this model assumes that the fluid flows once the local shear stress has exceeded the dynamic yield stress, xcfx84y, and the resulting viscous shear stress is additive and proportional to the strain rate, {dot over (xcex3)}, through the plastic or differential viscosity, xcexc. If the local shear stress is less than the dynamic yield stress, then the fluid does not flow, but is assumed to be rigid. Derivation of the damper force vs. velocity characteristic is the subject of Wereley and Pang (1998). The resulting discontinuity when transitioning across the zero shear rate condition leads to difficulties in dynamic modeling, but the Bingham-plastic model its more than adequate for design in the sense of predicting damping or energy dissipation of devices. A second problem with this model is that the post-yield viscosity is assumed to be constant, which is not the case in practice.
On the other hand, the Herschel-Bulkley model more accurately captures the post yield behavior of the fluid, in that the viscosity can vary as a fractional derivative of the shear rate as below
xcfx84=xcfx84ysgn({dot over (xcex3)})+K{dot over (xcex3)}nxe2x80x83xe2x80x83(3)
It should be noted that the preyield behavior of the Bingham-plastic and Herschel-Bulkley models is the same. Derivation of the damper force vs. velocity characteristic is the subject of Lee and Wereley (1999). The Herschel-Bulkley model can be expressed as a Bingham plastic model
xcfx84=xcfx84ysgn({dot over (xcex3)})+xcexca{dot over (xcex3)}xe2x80x83xe2x80x83(4)
where the apparent viscosity introduced here is now a function of shear rate
xcexca=K{dot over (xcex3)}nxe2x88x921xe2x80x83xe2x80x83(5)
This model is very useful in the analysis of dampers. The final model to be summarized is the Eyring model. This model has a constitutive equation of                     τ        =                                            1              K                        ⁢            sinh            ⁢                          xe2x80x83                        ⁢                                                                             -                  1                                            ⁢                              (                                                      γ                    .                                    ξ                                )                                              +                      μ            ⁢                          xe2x80x83                        ⁢                          γ              .                                                          (        6        )            
This model most accurately accounts for low strain rate behavior.
Based on rheometer tests performed in the Smart Structures Laboratory at Maryland and elsewhere, the Herschel-Bulkley performs slightly better over the range of shear rates ( greater than 30,000/second) that are of interest to this project.
All of the above models can be used to better predict fluid behavior and can be used as the basis for the analysis of dampers. However, it should be appreciated that additional terms must be added to the various models to accurately model the particular damper in question, such as: seal friction, bushing friction, nonlinear spring,effect of the pneumatic reservoir.
Referring to FIG. 6, an exemplary active MR damper 100 includes a cylinder 115, having a chamber 120 that contains an MR fluid. A piston 125 has a head 130, received in chamber 120, and a piston rod 135 extending from head 130 and through an aperture 140 in cylinder 115. A first spring retainer 155, connected to cylinder 115, and a second spring retainer 160, connected to piston rod 135, retain a recoil spring 165 that biases piston 125 relative to cylinder 115.
Referring also to FIG. 7, head 130 includes a bobbin 170 which retains one or more electric coils 175, each for selectably generating a magnetic field 180. A flux return 177, mounted on head 130, encircles and defines with bobbin 170 a fluid channel 185 configured to course the MR fluid between annular apertures 127 in head 130 through an active region or zone of influence 190 of magnetic field 180. When coil 175 energized, magnetic field 180 causes the MR fluid within active region 190 to assume a higher viscosity or resistence to flow, as described above. Piston 125 essentially xe2x80x9ctearsxe2x80x9d or shears the MR fluid as piston 125 moves relative to cylinder 115.
At least portions of bobbin 170 and flux return 177 which are influenced by magnetic field 180 should be, but as practical matter are entirely, constructed from a high permeability magnetic steel material that will not become permanently magnetized over time. Otherwise, coursing the MR fluid through a fluid channel defined by a magnetized structure would activate the MR fluid and diminish the viscosity range or ability to alter the viscosity thereof.
As shown, when bobbin 170 supports more than one coil 175, adjacent coils 175 are wound so as to generate adjacent active regions 190 having like polarity, thereby defining an enhanced active region.
A disadvantage of damper 100 is that significant portions thereof must be constructed from expensive high permeability magnetic steel material. Another disadvantage with damper 100 is that, with coils 175 fixed to piston 125, delicate electrical wires 178 that energize coils 175 reciprocate with piston 125, which may cause premature failure.
Some devices avoid both problems by fixing the coils in a relatively small fluid valve constructed from a high permeability magnetic steel material. See, for example, U.S. Pat. No. 5,993,358, issued Nov. 30, 1999, to R. S. Gureghian et al, entitled Controllable Platform Suspension System for Treadmill Decks and the like and Devices Therefor. However, such valves are contained in complex fluid systems, rather than in a conventional fluid damper. Also, such fluid systems also are not substantial enough for damping gun recoil forces.
MR damper control systems have been used to damp See, for example, U.S. Pat. No. 5,582,385, issued Dec. 10, 1996, to F. P. Boyle et al., entitled Method for Controlling Motion Using an Adjustable Damper; U.S. Pat. No. 5,964,455, issued Oct. 12, 1999, to D. M. Catanzarite et al., entitled Method for Auto-Calibration of a Controllable Damper Suspension System; and U.S. Pat. No. 6,311,110, issued Oct. 30, 2001, to D. E. Ivers et al, entitled Adaptive Off-State Control Method. However, none of these methods provide for managing energy dissipation, rather intend to eliminate the energy entirely.
To obtain more advantageous damping, gun dampers should provide variable damping for varying recoil energy dissipation as needed. To this end, damped gun systems should include variable dampers. Although a variable MR damper may be able to provide variable damping which more advantageously dissipates energy as needed, the damping provided also must be tailored to dissipate the specific energy associated with a particular round. To this end, the gun system should include an active damping system, wherein the damping of the MR damper is controlled based on the actual energy content of the round. What are needed, and not taught or suggested in the art, are an active, high-speed, high impulse damper and damping method.
The invention overcomes the disadvantages noted above by providing an active, high-speed, high impulse damper and damping method.
The invention provides a damper including a cylinder, a piston defining in the cylinder a volume, a coil, fixed relative to the cylinder, configured to generate a magnetic field, and a fluid channel, configured to be influenced by the magnetic field, for one or both of providing fluid to and evacuating fluid from the volume.
The invention also provides a damper including a cylinder, a piston defining in said cylinder a first volume and a second volume, a first fluid channel for one or both of providing fluid to and evacuating fluid from the first volume, first means for regulating flow through said first fluid channel, a second fluid channel for one or both of providing fluid to and evacuating fluid from the second volume, and second means for regulating flow through said second fluid channel, wherein said first fluid channel and said second fluid channel are in fluid communication.
The Invention further provides a method of damping with a damper, having a cylinder and a piston defining in the cylinder a first volume and a second volume, including causing negative fluid pressure to resist a tendency of the piston from increasing the first volume, and causing positive fluid pressure to resist a tendency of the piston from decreasing the second volume.
The invention additionally provides a gun system including a gun, a gun mount, and means for dissipating energy of a force exerted by the gun against the mount, wherein the means for dissipating is adjustable for dissipating different amounts of energy.
The invention yet also provides a control system for controlling recoil forces produced in an automatic rapid fire gun mounted on a support and having a variable damping characteristic mounted between the gun and the support, the damper employing an electrically or magnetically active working fluid. According to the invention, the fluid may have a viscosity characteristic which varies in response to an applied electric or magnetic field. In an exemplary embodiment, a damper is employed including such fluid having a variable viscosity characteristic responsive to an applied magnetic or electrical signal. The fluid exhibits a first viscosity characteristic when electrically or magnetically activated and exhibits a second viscosity characteristic lower than the first viscosity characteristic when deactivated. The viscosity characteristic varyies in accordance with the output levels of the applied signal. A force measuring sensor responsively coupled to the gun produces signal indicative of the recoil force of the gun. An electrical circuit responsively coupled to force measuring sensor and operatively coupled to the damper produces an output signal having a selected output level, operative for activating the fluid in accordance there, for varying in real time the viscosity characteristic of the fluid and thereby varying the damping characteristic of the damper.
The invention provides for reducing the number of coils needed in an MR damper, thereby reducing overall inductance in the associated magnetic circuit, thereby reducing the time constant of the circuit.
The invention also provides for reducing the overall number of turns in a coil to achieve the appropriate levels of magnetic field, thereby reducing the time constant and allowing faster MR fluid response. This reduces the complexity of manufacture and weight. The more efficient use of electrical power in the channel reduces the amount of power required it also allows the use of smaller coils which result in better response characteristics, in particular, with respect to the circuit time constant.
The invention provides for increasing the active length of a fluid channel by employing a C-shaped annular fluid channel which pneumatically amplifies the effectiveness of the device, thereby enabling a more compact design, and permitting the pneumatic reservoir to provide additional volume. Additional pneumatic reservoir volume, in turn, allows for a longer piston stroke and reduces the amount of expensive MR or ER fluid needed.
The invention provides for reducing weight and cost of an MR damper by substantially reducing the volume of high permeability magnetic steel required in activatable regions of the damper.
The invention provides for improving the mechanical force vs. velocity performance characteristics.
The invention provides improved elements and arrangements thereof, for the purposes described, which are inexpensive, dependable and effective in accomplishing intended purposes of the invention. Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments which refers to the accompanying drawings.